1.Field of the Invention
The field of the present invention relates to the field of batteries and of polymer electrolytes for batteries and more particularly to the field of lithium batteries.
More specifically, a subject matter of the present invention is a novel polymerizable and/or crosslinkable composition for a battery electrolyte, a novel polymer electrolyte obtained by polymerization and/or crosslinking of this novel composition, and a novel polymer battery.
2.Description of Related Art
Historically, lead batteries have been the most commonly used. However, there were numerous disadvantages to the lead technology, related to the weight of the batteries, to the unreliability during operation and to the use of a corrosive liquid. This led to the development of alkaline batteries, the electrodes of which were either based on nickel and cadmium (nickel—cadmium batteries), or based on zinc and nickel oxide (zinc—nickel batteries), or based on silver oxide coupled to zinc, cadmium or iron (silver oxide batteries). All these technologies use a potassium hydroxide solution as electrolyte and exhibit the major disadvantage of a low energy density by weight with regard to the requirements related to the development of portable devices. Thus it is that manufacturers have developed a new industry based on lithium batteries using a negative electrode based on lithium metal (hence the name “lithium metal battery”). However, problems related to poor restoration of the negative lithium electrode during successive chargings quickly resulted in a novel type of negative electrode based on carbon, used as insertion compound for lithium (hence the name “lithium ion battery”).
The operating principle for lithium batteries is summarized in the following way:
During the electrochemical charging, the transition metal ions of the positive electrode are oxidized, which results in the deintercalation of the lithium. Electrons are forced to move through the external circuit and a molar equivalent amount of lithium ions passes through the electrolyte, which is an ionic conductor and electronic insulator. This makes possible the intercalation of the lithium at the negative electrode. During the discharge of the battery, that is to say during use, it is the reverse phenomenon which occurs spontaneously.
In batteries, the ionic conductor or electrolyte, which separates the electrodes, is a key component. First, its state, liquid, solid or gelled, affects the safety of the system and, secondly, its conductivity determines the operating temperature range. Liquid electrolytes based on carbonates are commonly used. However, they do not exhibit the optimum safety conditions related to the handling of a corrosive liquid. This is because this type of battery can be the site of episodes, such as a thermal runaway, resulting in the formation of gas, thus increasing the internal pressure of the battery and the risk of explosion. It is for this reason that strict safety standards require manufacturers to use sophisticated cases, thus increasing the cost price of a unit.
In order to overcome this major disadvantage, the battery industry has developed a novel technology based on solid polymer electrolytes comprising a lithium anode, hence the name of “lithium polymer battery”. Due to its solid nature and being in the film form, this novel type of electrolyte makes possible the development of a safer battery having a great variety of shapes. The thinness of the films formed makes possible an increase in the energy efficiency at a low current density. One of the first “dry polymers” studied was polyoxyethylene for transportation applications. However, one of the disadvantages of this type of polymer is related to a low conductivity for use at ambient temperature and a fortiori at low temperatures. This is thus one of the major disadvantages which becomes critical for use of these batteries under extreme conditions, such as, for example, for geostationary satellite batteries operating in space.
The experts concerned have thus attempted to develop novel polymer electrolytes. By way of illustration, international application WO 2000/25323 discloses a composition which can be crosslinked to form a battery polymer electrolyte comprising a polysiloxane composed of polyoxyethylene groups or of cyclic carbonate groups having at least two reactive SiH groups, a crosslinking agent having at least two reactive groups of alkenyl type, a hydrosilylation catalyst and an electrolyte salt. This composition is crosslinked thermally by heating between 70 and 100° C. for a time of approximately 6 hours to produce an electrolyte polymer. The major disadvantages of this type of preparation are related to the high energy cost for manufacture of the electrolyte polymer and to a slow crosslinking rate which is a curb on industrial application.
The industries of the technical field under consideration are therefore waiting for novel compositions for a battery electrolyte which make it possible to obtain electrolyte polymers having sufficient levels of conductivity for use in a suitable temperature range extending from −20° C. to +80° C. and polymer electrolytes employing preparation routes with a low energy cost.